The present invention relates to apparatus for detecting normal, perforated, untreated postage stamps on envelopes in a machine, in which the letters are moved in one direction, comprising a light source for illuminating the envelopes at a small angle and a photoelectric scanning device.
U.S. Pat. No. 3,780,310, issued to H. Hudler, the applicant of the present application, describes an optical method by which normal, untreated postage stamps can be detected just as reliably as treated stamps can be detected by known methods. This is accomplished in accordance with the cited patent by the utilization of a feature which is common to all postage stamps, namely, the presence of perforations at the edge of the stamp. That feature can be utilized for detection because it constitutes a reliable code and the perforations can be provided by up-to-date machines with a substantially exact pitch. In the apparatus according to the cited patent, the light source is so arranged that the light rays make an acute angle with the plane in which the envelopes are conveyed so that the stamps are illuminated from the side with a scanning spotlight beam in such a manner that the sides of the teeth of the perforated edges are brightly illuminated and the evaluating device which controls a switching device can detect the illuminated edge of the postage stamp as an indication of the presence of the stamp.
The apparatus according to the cited patent is used to scan that perforated edge of the postage stamp which extends parallel to the direction of travel of the stamp and when the teeth of the postage stamps which succeed each other in the direction of travel move past the scanning device the latter generates a low-frequency pulse train, which indicates to the evaluating circuit the presence of a postage stamp. Because the location of the adhesively fixed postage stamp is not exactly known, it is necessary to scan a wide strip. For this reason, the scanning device consists of a relatively large number of photodiodes, which are connected in parallel to each other and to respective amplifier channels. During the scanning, a signal which indicates the perforate edge is transmitted only by one channel and the other channels may transmit interfering signals which may be due to folded edges or rough paper of the envelope or to similar causes. A considerable electronic expenditure is required to decrease this high susceptibility to interference.